1. Field of the Invention
The present invention relates to an optical disk drive for a disk that permits overwriting of a sector of the disk.
2. Description of the Prior Art
As the above optical disk (optical information recording media), a phase transition type system using a chalcogenide as a recording thin film material is known well. This recording thin film is a photo-sensitive layer formed on a substrate, which transfers into a crystal state or an amorphous state by irradiation with a light beam such as a laser beam. The recording layer of the overwritable optical disk of a phase transition type can record data usually by using a crystal state for a non-recorded state and an amorphous state for a recorded state. The amorphous state is generated by irradiating a laser beam to melt the recording layer followed by rapid cooling down thereof. The crystal state is generated for erasing the recorded data by irradiating a laser beam at a lower power to raise the temperature of the recording layer.
One of the merits of the phase transition type system is that only one laser beam is required and that the disk can be overwritten easily. If the laser beam power is modulated between two levels, i.e., a recording level and an erasing level, according to the write data, erasing old data and recording new data can be performed simultaneously by irradiating the modulated laser beam at the track of recorded data (Tokukaishou 56-145530). The phase transition type optical disk having the above merits is widely used for recording (i.e., writing) and reproducing (i.e.,reading) document files, picture image files and other data files.
The overwritable optical disk usually has a guide track in a spiral or in circles that is detected optically, for high density recording and for necessity of dispersed recording. The optical disk drive (i.e., optical disk reader/writer) irradiates a laser beam focused in a diameter of less than 1 .mu.m to the recording layer on the guide track of the optical disk for writing or reading of data.
It is common to divide a track into sectors to record variable length data effectively. Each sector that has usually a memory capacity of 512 bytes or 1024 bytes including the sector ID area including a track address and a sector address, and a recording area for writing and reading of data. An address portion that indicates a physical address of a sector is preformatted at the manufacturing stage.
Usually, a recording format for data recorded in the recording area includes a synchronizing signal (i.e., VFO) portion for drawing in of PLL (Phase Locked Loop), a data head indicating mark (i.e. DM) added to the head of the write data as a kind of synchronizing signal, a modulated data portion and a resynchronizing signal for word synchronizing. In the process of data recording, an address of a target sector ID is read and the data is written into the recording area of the target sector after detecting the address.
There are two recording methods, i.e., a pulse position modulation and a pulse width modulation. In the pulse position modulation, mark positions are detected for reading of data. On the other hand, both ends of marks are detected in the pulse width modulation. The pulse width modulation has an advantage in record density.
The recording and reproducing method of the phase transition type optical disk in the prior art is explained below, referring FIG. 14 and 15. FIG. 14 illustrates a block diagram of a reader/writer (optical disk drive) in the prior art. FIG. 15 shows write data, a laser power and a recording state of the optical disk for explaining the write/read operation.
As shown in FIG. 14, a system controller 4 connected to a host computer outputs the write information as a binary signal. This write information is provided with error correction information, then encoded in a encoder 7a with e.g., 1-7 RLL. A composer 8 adds a synchronizing signal (VFO) to each data block to be written into each sector so as to generate write data 11a. A laser power controller 12 controls the laser housed in a optical head 3 to modulate the intensity of the laser according to the write data 11a. The system controller 4 also controls a spindle motor 15 to rotate the optical disk.
If the strong laser beam (at laser power Pp) focused by the optical head 3 is irradiated to the recording layer of the optical disk 1 to raise the temperature of the recording layer above its melting point, the spot irradiated by the laser beam is melted and cooled rapidly, and assumes the amorphous state as a recorded mark 20. On the other hand, if the laser beam (at laser power Pb) is focused and irradiated to raise the temperature of the recording layer above the crystalization temperature but below the melting point, the recording layer at the irradiated spot assumes the crystal state. Data recording is performed in the above mentioned way using a difference between the crystal state and the amorphous state.
Data reproducing(reading) from the optical disk is performed using the difference of the optical character of the recording layer between the crystal state and the amorphous state. A weak laser beam (at laser power Pr) is focused and irradiated to the optical disk and a change of a reflected beam is detected as a read RF signal 14 of the recorded data. Then, the signal is converted to a binary signal in a read signal processor 13; it is further processed for decoding and error correction to be a desired read information 6. A similar weak laser beam is irradiated to the optical disk for reading address information when the beam scans the address portion between sectors 18 of the optical disk during the recording process.
However, it is known that repeated recording in a sector of the optical disk of the phase transition type may generate a deterioration that is unique to the phase transition type optical disk. This deterioration causes a reading error. The area of this type of deterioration usually spreads according to the number of times recordings is repeated. Three main patterns of the deterioration are as below:
(1) a defect of the recording layer is generated in a record start portion of a chain of recording areas due to the repeated recording; the defect spreads backward (direction of laser scanning on the disk); PA1 (2) a defect of the recording layer is generated in a record end portion of a chain of recording areas due to the repeated recording; the defect spreads forward (opposite direction of laser scanning on the disk); PA1 (3) a defect of the recording layer is generated in an area where the same pattern mark train is recorded; the defect spreads forward and backward.
Usually rewriting of the optical disk is performed by sectors. Therefore, the whole sector is rewritten even if the recorded data is only changed partly. Especially, a TOC (Table Of Content) area and a directory area of the disk are often written by similar data repeatedly. The above deterioration pattern (3) occurs in such areas.
It is understood that these three patterns of deterioration are all due to a slow migration of the material that forms the recording layer in the laser scanning direction or the opposite direction. However, what drives the recording layer material to migrate during laser irradiation is not known. Some driving forces are conceivable such as surface tension due to a thermal gradient in the recording layer during the laser irradiation or a deformation of the layers making up the optical disk due to a thermal load. If the recording layer has deteriorated, the necessary reflection of the laser beam corresponding to the recorded or non-recorded state of the optical disk cannot be obtained. Some recording methods are proposed to solve these problems so that the performance of rewriting improves. For example, as a solution of the above deterioration pattern (3), there is an optical disk drive that can suppress the deterioration by altering the start position of the VFO recording at every recording (Tokukaishou 63-229625). As a solution of the above deterioration pattern (2), there is an optical disk drive that can suppress the effect of deterioration of the waveform that spreads forward (direction toward the record start point) from the record end point by recording a fixed length of dummy data (Tokukaihei 2-297724).
In the above writing methods of the prior art, the start position of the VFO writing is altered at every writing, or the fixed length of dummy data is added to the end of the data block to suppress the deterioration of the read data due to the repeated recording. Therefore a dummy data area is necessarily added to the record area of the data block. This means that the recording capacity (byte number) is substantially decreased.